1. Field of the Invention
The present invention relates to an apparatus for reproducing a digital signal from a magnetic tape at a tape traveling speed different from that at which the digital signal was recorded into the magnetic tape.
2. Description of Related Art
A video tape recorder, referred to hereafter as a digital VTR, for converting an analog video signal into digital video data, compressing the amount of the data and recording as well as reproducing the compressed digital video data is under development. A simplified block diagram of a video-signal processing system employed in such a digital VTR is shown in FIG. 6. First of all, video signal processing in the digital VTR is explained by referring to this figure.
An input analog video signal is converted into digital video data by an A/D converter 1 before being supplied to a digital signal processing circuit 2. The digital signal processing circuit 2 carries out a shuffling process and data compression by using a DCT process and variable-length coding on the digital video data supplied thereto. A signal output by the digital signal processing circuit 2 is then supplied to a framing circuit 3. The framing circuit 3 frames the video data supplied thereto into a predetermined format and, at the same time, adds parity codes for error correction, synchronization codes and IDs. A signal output by the framing circuit 3 is subsequently sent to a channel encoder 4. The channel encoder 4 carries out predetermined recording modulation processing on the video data supplied thereto. Then, the output of the channel encoder 4 is amplified by a recording amplifier 5 before being recorded into a magnetic tape, not shown in the figure, by a transducer such as a magnetic head 6.
Video data reproduced in a playback operation from the magnetic tape, not shown in the figure, by a magnetic head 7 is amplified by a playback amplifier 8. A signal output by the playback amplifier 8 then undergoes demodulation, a counterpart of the recording modulation process described above, in a channel decoder 9 before being supplied to a sync/ID detecting circuit 10. The sync/ID detecting circuit 10 detects the synchronization codes and the IDs added to the video data and supplies them to a memory control circuit 11. The video data itself is supplied to a frame memory unit 12. At that time, only video data with an ID having an error flag set at a low level is stored in the frame memory unit 12. Here, error correction based on parity codes and a deframing process are carried out on the video data before being supplied to a digital signal processing circuit 13. The digital signal processing circuit 13 performs data decompression through decoding of the variable-length codes and a reversed DCT process on the video data supplied thereto. The digital signal processing circuits 13 further carries out a deshuffling process on the video data and supplies its output to a D/A converter 14. The D/A converter 14 converts the digital video data back into an analog video signal output.
A typical recording track pattern is shown in FIG. 7. As shown in the figure, in the case of a 525/60 system, a frame of a video signal is divided and recorded into 10 tracks. It should be noted that, in the case of a 625/50 system, a frame is recorded into 12 tracks. In addition, this track pattern is formed by a pair of heads having azimuth angles different from each other. The heads, which are referred to hereafter as two facing heads, are installed at positions on a circumference of a rotating drum separated from each other by an angle of 180 degrees. The track pattern can also be created as well by double azimuth heads installed at positions close to each other.
A relation between video data recorded on tracks and the screen is shown in FIG. 8. As shown in the figure, each one-tenth of the height of the screen from the top to the bottom of the screen is stored in a track.
Let us think of a playback operation in a VTR with a configuration described above with a tape traveling speed different from that at which video data was recorded. Such a playback operation is referred to hereafter as a variable-speed playback. As is generally known, during a variable-speed playback, the magnetic head moves along a plurality of recording tracks, reproducing a video signal. In addition, since adjacent tracks are used for recording data with azimuths opposite to each other, the waveform of an RF signal output by the playback amplifier 8 shown in FIG. 6 looks like balls of an abacus as shown in FIG. 9A. At that time, only a segment with an ID error flag of FIG. 9B set at a low level is stored in the frame memory unit 12 shown in FIG. 6. Such a segment is referred to as a valid data segment. Only video data of such a segment is used as an update of the frame memory unit 12. As a result, an updated picture is played back.
A pattern with an updated picture depends on the traveling speed of the magnetic tape. For example, in the case of a speed 9.0 times the normal traveling speed, an updated pattern like the one shown in FIG. 10 is created, resulting in a screen with portions thereof not updated at ordinary times. In the case of a 9.5-fold speed, on the other hand, an updated pattern like the one shown in FIG. 11 is created, reproducing video data of the entire screen. It is obvious that, by merely varying the speed of the magnetic tape even slightly, the appearance of the screen changes much. However, it should be noted that, even in the case of a 9.5-fold speed, an unupdated portion appears on the screen as shown in FIG. 12 if the acquisition rate of the data decreases. The acquisition rate is defined as a value indicating a level down from the peak of an RF output in terms of percents, up to which level the RF output can be acquired as valid data. If the traveling speed of the tape is shifted slightly to 9.48 times for example, however, updating becomes possible even though it is not possible at a 9.5-fold speed.
The above description indicates that it is necessary to select a tape speed so as to produce a screen that is easy to watch and includes no unupdated portions in order to sustain a good screen quality in a variable-speed playback operation.